Energy conservation in the presently existing social and industrial framework requires that increased attention be given to extensive coal resources as the available supplies of oil and gas from conventional sources are significantly diminished. Although many process schemes have been suggested and, in some instances, developed for the utilization of coal, environmental constraints have so far limited their consideration. The more capital intensive processes aim at providing a substitute for natural gas or for petroleum oils. Lower cost processes aim at gases having low to medium heating values. Often, such processes also yield a substantial amount of liquid fuel.
In the gasification of coal, heat is supplied largely by combustion of a portion of the coal. Typically, the combustion is effected by introducing air into the gasifying vessel. This necessarily leaves the inert gases from the air stream and from the combustion process in the product gas as a diluent, so that the heating value of the product fuel is markedly decreased. Additionally, environmentally undesirable oxides of nitrogen are produced. This effect can be avoided by the use of elemental oxygen, but this requires the addition of an oxygen plant and, of course, increases investment and operating costs. In another approach, coal particles are burned separately and the hot coke particles (from partial combustion) or ash particles (from substantially complete combustion) are introduced indirectly into the gasification vessel. This latter approach often requires very high combustion temperatures in the range of 3000.degree.-3500.degree. F.
Where multiple coal pyrolysis and gasification steps are employed, product gases generally provide heat for lower-temperature zones by countercurrent flow of product gases. In such instances, a char product, comprising up to about one-half of the carbon value of the coal feed, is usually produced.
The use of hot particles from a separate combustion zone as the heat source is described in U.S. Pat. No. 4,017,432, where coal char particles or extraneous solids particles may be employed as heat carriers. The use of countercurrent flow of gas for heating prior reaction stages is described, for example, in U.S. Pat. No. 3,375,175.
Most of the proposed gasification processes operate at elevated pressures, ranging from about 100 p.s.i.g. to over 3,000 p.s.i.g. Most low-pressure processes convert coal incompletely, providing coal char as a major product, or provide a gas having a low heating value.
There remains a need for a low-pressure low-temperature process which can effect a substantially complete conversion of carbon values in the coal while gasifying a maximum proportion of the carbon as well as the more-volatile coal components. There likewise remains a need for the effective removal of sulfur from product gas and combustion flue gas streams. Where these goals can be met, there will be provided the opportunity to employ conventional reaction vessels with simple metallurgical requirements. There will also be provided an economically attractive source of fuel gas to existing industrial locations presently faced with the need for extensive and expensive non-productive installations required in order to meet environmental standards, particularly for air and water quality.